Photoresist Gratings Research Articles

Brilliant quantum dots’ photoluminescence from a dual-resonance plasmonic grating

Semiconductor quantum dots (QDs) have recently caused a stir as a promising and powerful lighting material applied in real-time fluorescence detection, display, and imaging. Photonic nanostructures are well suited for enhancing photoluminescence (PL) due to their ability to tailor the electromagnetic field, which raises both radiative and nonradiative decay rate of QDs nearby. However, several proposed structures with a complicated manufacturing process or low PL enhancement hinder their application and commercialization. Here, we present two kinds of dual-resonance gratings to effectively improve PL enhancement and propose a facile fabrication method based on holographic lithography. A maximum of 220-fold PL enhancement from CdSe/CdS/ZnS QDs are realized on 1D Al-coated photoresist (PR) gratings, where dual resonance bands are excited to simultaneously overlap the absorption and emission bands of QDs, much larger than those of some reported structures. Giant PL enhancement realized by cost-effective method further suggests the potential of better developing the nanostructure to QD-based optical and optoelectronic devices.

Experimental band flip and band closure in guided-mode resonant optical lattices

We demonstrate band flip in one-dimensional dielectric photonic lattices presenting numerical and experimental results. In periodic optical lattices supporting leaky Bloch modes, there exists a second stop band where one band edge experiences radiation loss resulting in guided-mode resonance (GMR), while the other band edge becomes a nonleaky bound state in the continuum (BIC). To illustrate the band flip, band structures for two different lattices are provided by calculating zero-order reflectance with respect to wavelength and incident angle. We then provide three photonic lattices, each with a different fill factor, consisting of photoresist gratings on Si3N4 sublayers with glass substrates. The designs are fabricated using laser interferometric lithography. The lattice parameters are characterized and verified with an atomic force microscope. The band transition under fill-factor variation is accomplished experimentally. The measured data are compared to simulation results and show good agreement.

Full-color reflective filter in a large area exploiting a sandwiched metasurface.

Metasurface-based color filters show great potential in imaging devices and color printing. However, it is still a great challenge to meet the high demand for large-area flexible displays with structural color filters. Here, a reflective color filter is developed with a sandwiched metasurface, where the photoresist grating, complementary silver grating and silicon nitride grating are sequentially stacked on the substrate. Analytical results show that bandpass reflective spectra can be achieved due to the combined influence of guided mode resonance and cavity resonance, and full-spectrum colors including three primary colors can be generated by merely varying the period of the metasurface. With only photolithography and deposition technology involved, large-area samples incorporating pixelated metasurfaces are easily fabricated. Metasurfaces with three periods of 540 nm, 400 nm and 320 nm are experimentally obtained having peak reflective efficiency of ∼ 60%, demonstrating red, green and blue colors as theoretical results. A stripe sample with the structural period varying from 250 nm to 550 nm is fabricated in an area of 10 mm × 30 mm, displaying full-color reflections as simulated. Finally, with metasurfaces of three structural periods, the pixelated Soochow University logo is fabricated in a larger area of ∼ 30 mm × 30 mm. Therefore, the proposed structure shows high compatible to roll-to-roll nano-imprinting for large-area flexible displays, with the photoresist film can be easily substituted by UV film in addition.

High-repetition-rate and multi-pulse ultrashort laser damage of gold-coated photoresist grating

The gold-coated photoresist grating (GCPG) compressor is a core module of the high-repetition-rate superintense ultrafast lasers. Understanding how pulse trains with different repetition rates lead to laser-induced damage of GCPG is critical for future high-power laser applications. In this paper, we present the results of a multi-pulse laser-induced damage threshold (LIDT) investigation of GCPGs at different high repetition rates for the first time. The upper and lower endurance limits of S-on-1 beam-normal LIDT at 50–1000 Hz (800 nm, 35 fs) are ∼0.40 J/cm2 and 0.01 J/cm2, respectively, and the multi-pulse LIDT at 100 kHz is ∼0.02 J/cm2 corresponding to the peak intensity ∼64 GW/cm2 (1030 nm, 320 fs). The typical damage morphologies at 50–1000 Hz that the gold coating strips from the photoresist grating mask differ from those initiated from the expansion of the mask at 100 kHz. Focused ion beam scanning electron microscopy measurements show that the corners and small adhesion strength of interfaces are structural weaknesses. Moreover, theoretical modeling indicates that the differences mainly arise from pyrolysis and low heat-conduction of the mask. Overall, these results are highly practical and lay the foundation for the safe construction of high-repetition-rate ultra-high-power lasers based on the GCPG compressor.

Active control technology of a diffraction grating wavefront by scanning beam interference lithography.

To fabricate plain holographic gratings with high wavefront quality and to obtain the wavefront required in varied line-space grating, an active control technology of a diffraction grating wavefront by modulating the phase distribution of the scanning-beam interference lithography system was proposed. Sinusoidal wavefront control is simulated, and the controlled wavefront being almost the same as the target wavefront. A photoresist grating was fabricated whose surface is uniform and the wavefront is ideally sinusoidal. The theoretical analysis and experimental results confirmed that the wavefront of the diffraction grating can be actively controlled by modulating the phase distribution of the scanning-beam interference lithography system.

Subwavelength diffraction gratings with macroscopic moiré patterns generated via laser interference lithography.

We propose a simple and flexible fabrication approach based on the moiré effect of photoresist gratings for rapid synthesis of apodized structures with continuously varying depth. Minor modifications in a standard laser interference lithography setup allow creating macroscopic, visible by naked eye moiré patterns that modulate the depth of subwavelength diffraction gratings. The spatial frequency of this modulation is easily controlled in a wide range, allowing to create a quasicrystal in extreme cases. Experimental results are confirmed by a theory with clear graphical solutions and numerical modeling. The method is universal and does not depend on a specific choice of photoresist and/or substrate materials, making it a promising choice for structured light applications, optical security elements or as a basic structuring method of complex optical devices.

Patterning of high-aspect-ratio nanogratings using phase-locked two-beam fiber-optic interference lithography

Patterning high-aspect-ratio gratings by the phase-locked two-beam fiber-optic interference lithography (2-FOIL) is numerically and experimentally investigated in this paper. The Dill model is applied in the numerical simulation to understand the effects of an exposure dose and pattern contrast on the exposed photoresist grating profiles. Exposure experiments on the authors’ home-built 2-FOIL setup are conducted to demonstrate the suitability for manipulating the linewidth of photoresist gratings by tuning the exposure dose to achieve high aspect ratios over 6 at high pattern contrast thanks to the phase-locking mechanism. The high-aspect-ratio photoresist gratings serve as an excellent etching mask for the subsequent pattern transfer into underlying silicon substrates for high-aspect-ratio silicon gratings. Using these high-aspect-ratio silicon gratings as the nanoimprint mold, a square nanomesh is demonstrated by means of the multiple-step nanoimprint lithography. The authors’ work demonstrates that the proposed phase-locked 2-FOIL system enables high pattern contrast under long exposure duration, making it a suitable tool for fabricating high-aspect-ratio grating structures.

热压增大光刻胶光栅占宽比的方法及其应用

热压增大光刻胶光栅占宽比的方法及其应用

Metrology of Nanostructures by Tomographic Mueller-Matrix Scatterometry

The development of necessary instrumentation and metrology at the nanoscale, especially fast, low-cost, and nondestructive metrology techniques, is of great significance for the realization of reliable and repeatable nanomanufacturing. In this work, we present the application of a homemade novel optical scatterometer called the tomographic Mueller-matrix scatterometer (TMS), for the measurement of photoresist gratings. The TMS adopts a dual rotating-compensator configuration and illuminates the nanostructure sequentially under test conditions by a plane wave, with varying illumination directions and records. For each illumination direction, the polarized scattered field along various directions of observation can be seen in the form of scattering Mueller matrices. That more scattering information is collected by TMS than conventional optical scatterometry ensures that it achieves better measurement sensitivity and accuracy. We also show the capability of TMS for determining both grating pitch and other structural parameters, which is incapable by current zeroth-order methods such as reflectometry- or ellipsometry-based scatterometry.

Tunable polymer lasers based on metal-dielectric hybrid cavity.

Tunable distributed feedback polymer lasing was achieved in a metal-dielectric hybrid cavity. The laser device consisted of a double-layer grating structure and a polymer membrane. Interference lithography and oblique evaporation techniques were employed in fabricating the cavity. The photoresist grating was fabricated by interference lithography. Silver was obliquely evaporated onto the photoresist grating, forming a double-layer grating structure. Then a free-standing polymer membrane was attached on the structure. Under optically pumped conditions, low-threshold lasing was obtained, due to the plasmonic enhancement. The lasing wavelength can be tuned by changing the silver grating's thickness, which results from the variance of the effective refractive index of the cavity. These results can be used to design high-efficiency laser devices.

Surface‐Plasmon‐Polariton Diode by Asymmetric Plano‐Concave Nanocavities

AbstractHigh‐efficiency and direction‐controllable launching of surface plasmon polaritons (SPPs) is highly desired in plasmonic devices and photonic circuits. Here, a heteroplasmonic device is reported consisting of an array of asymmetric nanocavities with each nanocavity constructed by an asymmetric Ag nanoshell (AgNS) and a planar gold film (AuFilm). A photoresist grating is used both as a template for fabricating the AgNS array and as the dielectric filling medium of the nanocavities. Interaction between localized surface plasmon resonance in the asymmetric AgNSs and SPPs in the AuFilm has enhanced the propagating surface modes along the gold film. A phase modulation induced by the asymmetry and interference between the SPP modes have been the main physics supporting the directional propagation of the surface waves or the in‐plane optical diode effect at normal incidence. The SPP diode works in different directions at different wavelengths, which can be controlled by adjusting the asymmetry of the AgNSs.

Femtosecond laser induced damage of pulse compression gratings

Laser induced damage of Au-coated gratings (ACG) and metal multilayer dielectric gratings (MMDG) for pulse compression were measured using 800±35nm femto-laser with pulse width of 30.2fs. The -1st order diffraction efficiency of the ACG is over 90% in wavelength range from 700 to 1000nm. The MMDG has a 148nm bandwidth (750–897nm) with -1st order diffraction efficiency greater than 90%. The laser damage experiment on grating samples was performed in air for single-shot damage. The single-shot damage threshold of the ACG and MMDG was determined to be 0.32±0.02J/cm2 and 0.31±0.02J/cm2, respectively. The damage morphologies of the ACG revealed that the damage was attributed to the pinholes at the base of the grating pillars and theweak adhesion between metal layer and photoresist gratings layer. The damage feature combined with near field distribution of MMDG indicated that the damage was due to the nonlinear ionization process of the valence electrons in HfO2 film. According to analysis results, the laser damage resistance of the ACG can be enhanced through avoiding the appearance of pinholes and increasing adhesion between metal layer and photoresist layer. And for the MMDG, good performance of HfO2 film, low near field enhancement and single HfO2 grating structures may increase its laser damage resistance.

Ultrashort pulses-driven dynamics of blisters in Au-coated gratings

Au-coated gratings (ACGs) for chirped pulse compression were irradiated using 800 ± 35 nm multiple laser pulses with duration of 30 fs. The laser-induced damage threshold (LIDT) of Au-coated gratings decreases from 0.35 J/cm2 to 0.10 J/cm2 with the increased pulse shots. The near threshold damage feature of Au-coated gratings at different pulses shows blisters. It is indicated that the gradually increased blister height with increasing multiple pulses on ACGs is caused by the accumulation effect. The damage morphologies combine with simulated results indicate that the damage of the ACGs can be attributed to the pinholes at the base of grating pillars and the separation of gold coating and photoresist gratings caused by the photoresist absorbed the incident laser energy. Therefore, we consider the damage threshold can be enhanced by optimizing coating deposition technology to reduce pinholes density and increase the adhesion between the gold and the photoresist.

Structural color analysis of the photoresist grating influenced by the light source and its parameters.

The basic principle and method to generate structural color from the photoresist grating of the multilayer dielectric diffraction grating are introduced. Rigorous coupled-wave analysis is used for computation with the open software RCWA-1D. The relationships between the characteristics of the photoresist and the structural color are explained. This paper discusses the effect of light source characteristics on the duty cycle color resolution, indicating that TE polarization is better than TM polarization, and the FWHM should be sufficiently large with an optimized value for the incident angle.

Directional elastic wave propagation in high-aspect-ratio photoresist gratings: liquid infiltration and aging.

Determination of the mechanical properties of nanostructured soft materials and their composites in a quantitative manner is of great importance to improve the fidelity in their fabrication and to enable the subsequent reliable utility. Here, we report on the characterization of the elastic and photoelastic parameters of a periodic array of nanowalls (grating) by the non-invasive Brillouin light scattering technique and finite element calculations. The resolved elastic vibrational modes in high and low aspect ratio nanowalls reveal quantitative and qualitative differences related to the two-beam interference lithography fabrication and subsequent aging under ambient conditions. The phononic properties, namely the dispersion relations, can be drastically altered by changing the surrounding material of the nanowalls. Here we demonstrate that liquid infiltration turns the phononic function from a single-direction phonon-guiding to an anisotropic propagation along the two orthogonal directions. The susceptibility of the phononic behavior to the infiltrating liquid can be of unusual benefits, such as sensing and alteration of the materials under confinement.

Duty cycle estimate of photoresist gratings via monitoring TM/TE diffraction efficiency ratio during development.

We present an in situ duty cycle control method that relies on monitoring the TM/TE diffraction efficiency ratio of the -1st transmitted order during photoresist development. Owing to the anisotropic structure of a binary grating, at an appropriately chosen angle of incidence, diffraction efficiencies in TE and TM polarizations vary with groove depth proportionately, while they vary with duty cycle differently. Thus, measuring the TM/TE diffraction efficiency ratio can help estimate the duty cycle during development while eliminating the effect of photoresist thickness uncertainty. We experimentally verified the feasibility of this idea by fabricating photoresist gratings with different photoresist thicknesses. The experimental results were in good agreement with theoretical predictions.

Direct Transfer of Metallic Photonic Structures Onto End Facets of Optical Fibers

We present a flexible approach to transfer metallic photonic crystals (MPCs) onto end facets of optical fibers. The MPCs were initially fabricated on a glass substrate with a spacer layer of indium tin oxide (ITO), which was used as a buffer layer in the transferring process. The fiber ends were firstly welded on the top surface of the MPCs by a drop of polymer solution after the solvent evaporated. The ITO layer was then etched by hydrochloric acid (HCl), so that the MPCs got off the substrate and were transferred to the fiber ends. Alternatively, the MPCs may be also etched off the substrate first by immersing the sample in HCl. The ultra-thin MPC sheet consisting of gold nanolines interlaced with photoresist gratings was then transferred to cap the fiber ends. In the later approach, we can choose which side of the MPCs to be used as the contact with the fiber facet. Such methods enabled convenient nanostructuring on optical fiber tips and achieving miniaturized MPC devices with compact integration, extending significantly applications of MPCs. In particular, the fabrications presented in this manuscript enrich the lab-on-fiber engineering techniques and the resultant devices have potential applications in remote sensing and detection systems.

Directional crystallization of polymer molecules through solvent annealing on a patterned substrate.

Multi-crystalline fibers of poly (3-hexylthiophene) (P3HT) have been grown on a patterned substrate through annealing the spin-coated film in the solvent of 1,5-pentanediol at 120 °C. The large-area patterns of photoresist grating have been fabricated by interference lithography, enabling growth of crystal fibers in a length scale of centimeters. The produced organic crystal fibers were characterized by microscopic, spectroscopic, and electrical conductivity performances. Mechanisms for the crystallization processes were investigated.

A novel method to fabricate silicon tubular gratings with broadband antireflection and super-hydrophobicity.

We have developed a novel method to fabricate micro/nano structure based on the coherent diffraction lithography, and acquired periodic silicon tubular gratings with deep nano-scale tapered profiles at the top part. The optical properties of these tubular gratings were similar to an effective gradient-index antireflective surface, resulting in a broadband antireflective combining super-hydrophobic behavior. The mechanism of the method was simulated by rigorous coupled wave analysis algorithms. Then coherent diffraction lithography by use of suitable mask, in which periodic micro-scale circular opaque patters were distributed, was realized on the traditional aligner. Due to coherent diffraction, we obtained enough light intensity for photoresist exposure under the center of the opaque area in the mask together with transparent areas. The tapered line profiles and hollow photoresist gratings over large areas could be fabricated on the silicon wafer after development. The dry etching process was carried out, and high aspect ratio silicon tubular gratings with deep tapered profiles at the top were fabricated. The optical property and wettability of the structure were verified, proving that the proposed method and obtained micro/nano structure provide application potential in the future.

Improved measurement accuracy in optical scatterometry using correction-based library search

Library search is one of the most commonly used methods for solving the inverse problem in optical scatterometry. The final measurement accuracy of the conventional library search method highly depends on the grid interval selected for each parameter in the signature library, and the time cost of the parameter extraction increases dramatically when the grid interval is decreasing. In this paper, we propose a correction-based library search method to improve the measurement accuracy for a pregenerated signature library. We derive a formulation to estimate the error between the expected solution of the inverse problem and the actually searched solution obtained by the conventional library search method. Then we use the estimate of the error as a correction term to correct the actually searched solution to improve the measurement accuracy. Experiments performed on a photoresist grating have demonstrated that the proposed correction-based library search method can achieve much more accurate measurement with negligible computational penalty to the conventional library search method in the parameter extraction. It has also been observed that the correction-based library search method has higher measurement accuracy and less time cost than the interpolation-based library search method. The proposed correction-based library search method is expected to provide a more practical means to solve the inverse problem in state-of-the-art optical scatterometry.